Each crude oil type has unique molecular characteristics. Crude oil assaying is the physical and chemical evaluation of crude oil feedstock by petroleum testing laboratories. Assays vary considerably throughout the industry, from just a handful of key properties for the whole crude to a full set of physical, chemical, and chromatographic measurements on distilled or blended fractions and residues of the crude oil. See M. Unavane, Statistical Tools for Managing and Manipulating Crude Oil Data, Exploration & Production, 2010, 8:135-138 (hereinafter “Unavane”). Results of crude oil assay testing provide extensive hydrocarbon analysis data for refiners, oil traders and producers. For example, assay data help refineries determine if a specific crude oil feedstock is compatible with a particular petroleum refinery or if the crude oil could cause yield, quality, production, environmental, or other problems.
The determination of a detailed crude oil assay is a lengthy, tedious and costly process. See M. Watt, S. Roussis, Crude Assay, Practical Advances in Petroleum Processing, Chapter 3, 103-116 (2006) (hereinafter “Watt”). The conventional approach to perform an assay consists of a limited set of measurements on the crude oil and its fractions. See Unavane. Most often only a few boiling points, densities, and other physical or chemical property measurements are available for selected distilled fractions or the whole crude oil. Densities are typically reported in American Petroleum Institute (API) gravity degrees. API gravity is a measure of how heavy or light petroleum liquid is compared to water, which has a defined API gravity of 10 degrees. A crude oil having a higher API gravity is lighter (less dense) than water. See Oil & Gas Journal Databook, 2006 Edition, PennWell Corporation, Tulsa, Okla., 2006 (hereinafter “Oil & Gas Databook”).
Due to limited available data, it is necessary for crude oil assay experts to predict or estimate missing properties to meet various business needs, such as refinery planning and scheduling and refinery process simulation. Typically, lower order polynomial expressions are used for interpolation, and an arithmetic probability function is used for extrapolation of boiling point curves. See Aspen Technology, Inc., Aspen HYSYS, v7.3, Burlington, Mass. Analytical approaches include the prediction of crude oil properties by correlating the data obtained by rapid surrogate measurements (typically spectroscopic measurements) to existing crude assays. See Watt; J. M. Brown, Method for Analyzing An Unknown Material as A Blend of Known Materials Calculated So As to Match Certain Analytical Data and Predicting Properties of the Unknown Based on the Calculated Blend, U.S. Pat. No. 6,662,116 B2, Dec. 9, 2003. See also J. M. Brown et al., Estimating Detailed Compositional Information from Limited Analytical Data, US Patent Application Publication US2009/0105966 A1, Apr. 23, 2009.
Statistically derived predictive methods have also been extensively used in the industry for the prediction or estimation of crude oil properties and assays. See Unavane. The issues with statistical methods are 1) in cases data are scarce or sporadic, it may not be possible to develop statistically meaningful models, 2) in cases data are abundant but with inconsistent data quality including high uncertainties, the quality of models or correlations can be questionable, 3) in cases data are abundant and with adequate quality, the complexity of petroleum mixtures still makes the statistical model development very difficult and highly subjective especially when developing interrelationships between different properties, and 4) in cases data are not available, there is nothing statistical methods could offer.
Crude oil assays are often used to generate a limited number of “pseudocomponents” or “hypothetical components” and their compositions are then used to represent petroleum mixtures for the purpose of planning, scheduling, and process simulation. These “pseudocomponents,” derived from the true boiling point (TBP) characterization curve as “micro boiling point cuts,” are not real hydrocarbon components or molecules in the petroleum mixtures. See M. R. Riazi, Characterization and Properties of Petroleum Fractions, 1st Ed., ASTM, West Conshohocken, Pa., USA, 2005, p. 111-112. These pseudocomponents are defined to mimic a fraction of the crude assay, that is, the pseudocomponent is defined by a specific boiling point range and by a range of specific gravity, or by a group of pure components from a hydrocarbon family with a specific range of carbon number. In contrast, a petroleum fraction (e.g., fuel oil) is a mixture of real hydrocarbon molecules obtained from fractional distillation of crude oil. A disadvantage of using pseudocomponent is that their physical and chemical properties need to be estimated by often inadequate empirical methods.
Therefore, there is a need for an improved crude oil assay method for estimation of crude oil properties and assays.